Dispersive optical elements disperse light by deviating the path of light passing through them by an amount that varies with wavelength. Prisms and gratings are two types of dispersive optical elements. In particular, prisms disperse light because their geometry causes light of different wavelengths passing through them to be separated and deviated by different amounts. In diffraction gratings, light passing through the grating is diffracted into a series of orders caused by the interference of wavefronts emitted from each slit in the grating.
Dispersive optical elements are used in spectrometers. Spectrometers are used in fields including medicine, material sciences, chemistry, environmental sciences, and so on. Spectrometers use diffractive and refractive dispersive optical elements including gratings and/or prisms to facilitate analyzing the spectral composition of sampled light.
As illustrated in FIG. 1, a conventional spectrometer 110 includes a light input entrance element 112 (Detector arm fiber) for receiving light from an external source. More generally the light entrance element 112 may be referred to as an electromagnetic radiation entrance. The light is passed through a first set of collimating optics 114 (collimating lens) to a diffraction grating 116. The diffraction grating 116 separates the light into various spectra. The separated light passes through a second set of focusing optics 118 to a detection element 120 (detector).
Fourier-domain optical coherence tomography (FDOCT) uses a Fourier transform of spectrum to calculate distribution of scatterers in a sample. Digital Fourier transform algorithms typically rely on data that is evenly spaced in frequency. However, gratings (for example, diffraction grating 116) disperse spectrum proportional to wavelength, not frequency. Thus, as illustrated in FIG. 2A, even spaced frequencies do not have even spacing on the detector 220 in a conventional spectrometer 210. Thus, as illustrated in the graph of FIG. 2B, conventional spectrometers 210 may experience about a 50% change in dispersion across a full spectrum.
U.S Patent Application Publication No. 2009/0040521 entitled EVEN FREQUENCY SPACING SPECTROMETER AND OPTICAL COHERENCE TOMOGRAPHY DEVICE to Hu et al. addresses this relatively high change in dispersion by a prism air-spaced with respect to a grating. This is also discussed in FOURIER DOMAIN OPTICAL COHERENCE TOMOGRAPHY WITH A LINEAR-IN-WAVENUMBER SPECTROMETER by Hu et al. Hu discusses using first and second dispersive elements, for example, a grating and a prism, separated by an air gap to approximately linearize the dispersion angle as a function of wavenumber.
However, improved systems for reducing the overall change in dispersion may be desired.